An image forming apparatus using an electrophotographic method, such as a copier, a printer or a multi-functional peripheral (MFP), transfers a toner image formed on a photoconductive drum onto a sheet so as to perform printing. The transfer is performed in a transfer position near the outer surface of the photoconductive drum. A sheet before printing is contained in a sheet cassette or the like, and the sheet is picked up from the sheet cassette upon printing and is guided to the transfer position via a transportation path composed of several rollers and the like.
While the sheet is transported from the sheet cassette to the transfer position, the sheet may be skewed. If the skew occurs, a line of a front end of the sheet is not parallel to a direction perpendicular to a transportation direction of the sheet and the sheet is transported with any angle. This angle may be called a skew angle or a skew amount. A state in which the skew angle is zero is an ideal state. When the toner image is transferred from the photoconductive drum to the sheet in a state in which the skew occurs, an inclined image is printed on the sheet and it is unfavorable.
Generally, a pair of rollers for correcting the skew, such as registration roller, is provided just before the transfer position. The rotation of the registration roller is stopped at a time point when the printing is instructed. The front end of the sheet which is transported from the transportation path is brought into contact with a nip of the registration roller of which the rotation is stopped. If the skew occurs in the sheet, one end of the front end line of the sheet is brought into contact with the nip first. Thereafter, the sheet is transported from the transportation path and deflection occurs in the vicinity of the end of the sheet which is in contact with the nip first. Then, the other end of the sheet also reaches the nip and the front end line of the sheet is parallel to the line of the nip. The deviation in the front end line of the sheet is absorbed by the deflection occurring in the vicinity of the end of the sheet so as to correct the skew. When it is determined that the skew has been corrected and the front end line of the sheet becomes parallel to the line of the nip (that is, when it is determined that the correction of the skew is completed), the rotation of the registration roller is started and the front end line of the sheet is transported to the transfer position through the nip of the registration roller.
The completion of the skew correction is determined whether a predetermined period (skew correction period) is elapsed after the front end of the sheet is brought into contact with the registration roller. If the skew correction period is too short, sufficient deflection necessary for the skew correction cannot be obtained and thus the correctable skew amount can be decreased.
On the other hand, if the skew correction period is set to be long, the correctable skew amount is increased, but a period from a time point when the front end of the sheet is brought into contact with the registration roller and a time point when the rotation of the registration roller is started is increased. Thus, printing throughput deteriorates. In addition, if the skew correction period is set to be long, a margin of the transportation period which has been set in order to print one sheet is decreased and thus a paper jam is easy to occur. Further, an unnecessarily large deflection occurs in a sheet with a small skew amount so as to cause paper wrinkle.
In order to solve these problems, a technique of detecting a skew amount before a sheet reaches registration roller and adjusting a skew correction period according to the detected skew amount is disclosed in JP-A 2005-350155.
In the technique disclosed in JP-A 2005-350155, the skew amount is detected using an edge sensor for detecting a side edge of the sheet and the sheet transportation amount (skew correction period) is adjusted according to the detected skew amount.
However, since the edge sensor is used for detecting the skew amount in the technique disclosed in JP-A 2005-350155, the sheet transportation amount needs to be changed according to the sheet size (sheet width) of a side direction and thus a process becomes complicated. In order to perform a function for detecting the edge, a predetermined gap needs to be provided between the edge sensor and the registration roller and a position where the edge sensor is provided is restricted. In addition, in the technique disclosed in JP-A 2005-350155, since the detected skew amount is influenced by the sheet transportation period, the skew amount cannot be accurately detected if the transportation period is changed by abrasion of a transportation roller located on the sheet transportation path.